1. Field of the Invention
The present invention relates to the process of setting a calcined gypsum slurry, and is more particularly concerned with a composition for accelerating the setting of an aqueous calcined gypsum slurry, such as in the formation of gypsum wallboard.
When freshly calcined gypsum, usually referred to as calcium sulfate hemihydrate or plaster of Paris, is mixed with water to a plastic consistency, it combines with some of the water to form calcium sulfate dihydrate and sets to a hard solid mass in about 30 minutes without the addition of set-influencing additives. The setting time, however, changes with the age of the plaster after its manufacture. After a very short interval, the setting time first begins to lengthen and then generally speeds up in an erratic and unpredictable fashion.
Gypsum board has long been a large volume commercial article of commerce. In the manufacture of gypsum board, a calcined gypsum aqueous slurry and desired additives are blended in a continuous mixer, as for example described in U.S. Pat. No. 3,359,146. The mixed slurry is continuously deposited upon a cover sheet moving beneath the mixer and a second cover sheet is applied thereover. The sandwiched slurry, as a continuous strip, is conveyed on a belt until the calcined gypsum has set; then, the strip is cut to form boards of desired length which are conveyed through a drying kiln to remove excess moisture. The time at which the board may be cut, or in other words the speed of the conveyor and the consequent rate of production of the gypsum board, is generally controlled by the setting time of the calcined gypsum slurry. Thus, conventional adjuvants to the calcined gypsum in the mixer generally include set time control agents, particularly accelerators. These and other additives such as pregenerated foam to control final density of the board, cover sheet bond promoting agents, fibrous reinforcements, consistency reducers and the like constitute less than 5%, and usually less than 2%, of the weight of the finished board core.
In a related area of manufacture, formulated building plasters of calcined gypsum are often bagged while still at elevated temperatures immediately following the calcination of the gypsum. Under these conditions, many set control additives deteriorate, so that they do not perform reliably. Storage conditions, particularly humidity and temperature, further adversely affect the performance of some accelerators.
2. Prior Art
The most common accelerator used for reducing the setting time of calcined gypsum is calcium sulfate dihydrate that has been ground to a high degree of fineness. When freshly prepared, it has high potency. However, when stored prior to use, particularly in the presence of moisture or heat, it loses its effectiveness. King in 1935 disclosed a way around this, in U.S. Pat. No. 1,989,641. He there proposed adding to the calcined gypsum certain amounts of lime and aluminum sulfate, along with a commercial retarder, to generate in situ a precipitated calcium sulfate dihydrate to act as seeding agent to accelerae the set of the plaster. Thereafter, King disclosed in U.S. Pat. Nos. 2,078,198 and 2,078,199 improved accelerators, comprising calcium sulfate dihydrate intermixed with sugar, which were less subject to aging deterioration. U.S. Pat. No. 3,573,947 subsequently disclosed a method for improving the sugar-coated accelerator employing a final finishing step of heating the co-ground sugar and calcium sulfate dihydrate mixture under conditions which permit a certain amount of calcination to take place to further stabilize the accelerator.
While all of these patents call for samll amounts of the accelerator to be used, as little as 1 pound per ton of plaster, about 20 pounds per ton or more is preferred in commercial manufacturing operations. Individual gypsum deposits vary in purity and the need for an accelerator correspondingly varies on different plant equipment, with some calcined gypsums requiring much larger amounts than others to maintain gypsum board line speed. Thus, there remains a need for an improved accelerator which has a higher degree of activity per unit of weight, may use less sugar, and exhibits little change in accelerative characteristics when exposed to storage, particularly high relative humidities and high temperatures. Thus, it is an object and advantage of the present invention to provide an accelerator for calcined gypsum which is extremely active in that it induces considerably faster set acceleration, is extremely efficient in that lessened amounts may be used to achieve the same levels of effectiveness as prior accelerators, and is storage stable. Other objects and advantages will become apparent from the ensuing description.